Method and apparatus for transmitting and receiving data using multi-user superposition coding in a wireless relay system

ABSTRACT

A method and apparatus for joint unicast using multi-user superposition coding in a wireless relay system are provided. A BS superposition-encodes first and second data messages directed to first and second MSs, respectively, scheduled at a current scheduling instant. The first and second data messages carry first and second information bit streams for the first and second MSs, respectively. The first MS has a relatively good direct link to the BS, and the second MS has a relatively bad direct link to the BS. The superposition-coded data is transmitted to the first MS and an RS connected between the BS and the first and second MSs. The RS receives the superposition-coded data from the BS, extracts the second information bit stream by decoding the superposition-coded data, and transmits a third data message carrying the second information bit stream to the first and second MSs.

PRIORITY

This application claims priority under 35 U.S.C. §119(a) of a KoreanPatent Application filed in the Korean Intellectual Property Office onMar. 16, 2007 and assigned Serial No. 2007-26114, the entire disclosureof which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to data transmission andreception in a wireless relay system, and more particularly, to a methodand apparatus for joint unicast using Multi-User Superposition (MUS)coding.

2. Description of the Related Art

One active research area of a future generation communication system,such as the 4^(th) Generation (4G) communication system, involvesproviding a large amount of data with various Quality of Service (QoS)requirements at high rates to users. To enable high-speedcommunications, the 4G communication system uses cells having very smallradii. This means that a conventional centralized wireless networkdesigning scheme is not viable for implementation of the system. In thiscontext, there is a need for a wireless network design that supportsdistributed control and actively copes with changes in a cellenvironment such as additional installations of Base Stations (BSs).Hence, a self-configurable wireless network is required whichautonomously configures a wireless network without control of a centralsystem in a distributed fashion to provide communication services.

Techniques used for an ad hoc network should be adopted to deploy theself-configurable network in the 4G communication system. A major ad hocnetwork is a multi-hop relay cellular network realized by employing amulti-hop relay scheme to a cellular network system including fixed BSs.Typically, the BSs are fixed in position in the cellular network. Theresulting less flexibility in configuring a wireless network makes itimpossible to provide efficient communication services in a wirelessenvironment experiencing fluctuating changes in traffic distribution orthe number of required calls.

In the 4G communication system, to avert this problem, theself-configuration wireless network uses a relay scheme in which data isdelivered over multiple hops through a plurality of neighboring MobileStations (MSs) and fixed Relay Stations (RSs), to thereby enable fastnetwork reconfiguration according to environmental changes and enableefficient operation of the overall wireless network.

The multi-hop relay wireless network offers the benefits of cellcoverage expansion and increased system capacity. When the channelstatus between a fixed BS and an MS is poor, a multi-hop link isestablished between them via an RS so that a better radio channel isprovided to the MS. Efficient communication services can be provided,especially in a shadowing area with a severe shielding effect caused bybuildings.

The RS-based wireless relay system needs to transmit data from a BS toan intended MS efficiently on the downlink. There exists a need for atechnique for increasing overall communication efficiency, especiallywhen the BS serves a plurality of MSs and an RS assists downlinktransmissions for MSs having poor direct links to the BS.

SUMMARY OF THE INVENTION

The present invention has been made to address at least the aboveproblems and/or disadvantages and to provide at least the advantagesdescribed below. Accordingly, an aspect of the present inventionprovides a method and apparatus for joint-unicasting data to multipleusers using superposition coding in a wireless relay system.

Another aspect of the present invention provides a method and apparatusfor transmitting data sets directed to different users by superpositioncoding from a BS in a wireless relay system.

A further aspect of the present invention provides a method andapparatus for establishing a multi-hop link between a BS and an MS viaan RS in addition to a direct link between them in a wireless relaysystem.

According to one aspect of the present invention, a method fortransmitting data via an RS in a wireless relay system is provided. A BSencodes a first data message and a second data message directed to afirst MS and a second MS scheduled at a current scheduling instant bysuperposition coding. The first data message carries a first informationbit stream for the first MS. The second data message carries a secondinformation bit stream for the second MS. The first MS has a relativelygood direct link to the BS, and the second MS has a relatively baddirect link to the BS. The superposition-coded data is transmitted tothe first MS and an RS connected between the BS and the first and secondMSs by the BS. The RS receives the superposition-coded data from the BS,extracts the second information bit stream for the second MS by decodingthe superposition-coded data, and transmits a third data messagecarrying the second information bit stream to the first and second MSs.

According to another aspect of the present invention, a method forreceiving data via an RS in a wireless relay system is provided. A firstMS receives superposition-coded data including a first data messagecarrying a first information bit stream for the first MS and a seconddata message carrying a second information bit stream for the second MS.The first MS has a relatively good direct link to a BS, which isscheduled along with the second MS having a relatively bad direct linkto the BS at a current scheduling instant by a BS. A third data messagecarrying the second information bit stream directed to the second MS isreceived from an RS connected between the BS and the first and secondMSs. The second information bit stream is extracted by decoding thethird data message. The first data message is acquired by removingcomponents related to the second information bit stream from thesuperposition-coded data. The first information bit stream is extractedby decoding the first data message.

According to a further aspect of the present invention, an apparatus fortransmitting and receiving data via an RS in a wireless relay system isprovided. A BS superposition-encodes a first data message carrying afirst information bit stream for a first MS and a second data messagecarrying a second information bit stream for a second MS having arelatively bad direct link to the BS. The first and second MSs arescheduled at a current scheduling instant. The superposition-coded datais transmitted. An RS connected between the BS and the first and secondMSs establishes multi-hop links, receives the superposition-coded datafrom the BS, extracts the second information bit stream for the secondMS by decoding the superposition-coded data, and transmits a third datamessage carrying the second information bit stream to the first andsecond MSs. Herein, the first MS receives the superposition-coded datafrom the BS, receives the third data message from the RS, extracts thesecond information bit stream by decoding the third data message,acquires the first data message by removing components related to thesecond information bit stream from the superposition-coded data, andextracts the first information bit stream by decoding the first datamessage.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptionwhen taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating RS-assisted scheduling and transmissionin a cellular system according to an embodiment of the presentinvention;

FIG. 2 is a diagram illustrating a downlink transmission scenarioaccording to an embodiment of the present invention;

FIG. 3 is a flowchart illustrating a joint unicast operation accordingto an embodiment of the present invention;

FIG. 4 is a diagram illustrating a message flow for an operation among aBS, an RS and MSs, according to an embodiment of the present invention;

FIGS. 5 and 6 are graphs comparing the present invention withconventional methods in communication performance, when equal timeresources are allocated to two MSs;

FIGS. 7 and 8 are graphs comparing the present invention with theconventional methods in communication performance, when transmissionrates are allocated at a predetermined ratio to two MSs; and

FIGS. 9 and 10 are graphs comparing the present invention with theconventional methods in communication performance, when equaltransmission rates are allocated to two MSs.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are described in detailwith reference to the accompanying drawings. It should be noted thatsimilar components are designated by similar reference numerals althoughthey are illustrated in different drawings. Detailed descriptions ofconstructions or processes known in the art may be omitted to avoidobscuring the subject matter of the present invention.

The embodiments of the present invention efficiently transmit downlinkdata from a BS to MSs using superposition coding in a wireless relaysystem having an RS between the BS and the MSs.

The wireless relay system uses a plurality of RSs in order to increaseservice coverage and system throughput and reduce communication delay.When the BS serves multiple MSs, an RS assists downlink transmissionsfrom the BS for some users, which in general have poor links to the BS.If an MS has a good direct link to the BS, then the RS is not needed forthe MS. A good link refers to a link having a Signal-to-Noise Ratio(SNR) that ensures an error rate equal to or below a required thresholdwhen a user receives data. In the case where the direct link of an MS isof intermediate quality, the MS makes an effective choice between thelink to the BS and the link to the RS. For such an intermediate case, arelaying method based on superposition coding is used to increasespectral efficiency.

In a multi-user system, transmission resources are usually time,frequencies, or codes. As data is transmitted simultaneously to aplurality of receivers in the same transmission resources or as aplurality of transmitters transmit data simultaneously in the sametransmission resources, system capacity can be significantly increased.For this simultaneous information transmission in the same transmissionresources, superposition coding is used.

According to the embodiments of the present invention, superpositioncoding is used for downlink transmission in a wireless relay systemusing RSs. When a direct link is impossible between a BS and an MS dueto factors such as shadowing, the BS and the MS are connected via an RS.A plurality of RSs can be used depending on the number of MSs served bythe BS.

FIG. 1 illustrates RS-assisted scheduling and transmission in a cellularsystem according to an embodiment of the present invention. In theillustrated case of FIG. 1, a BS serves only a single user on a singlechannel at each scheduling instant. This channel is used in atime-division manner by the BS and an RS.

Referring to FIG. 1, the BS selects a single MS, MS_(k) or MS₁ accordingto some criteria, for example, maximal throughput, maximal proportionalfairness, etc. at each of first and second scheduling instants 102 and104 and transmits data to the selected MS either directly or with thehelp of the RS. If the RS needs to assist the downlink transmission, ittransmits data to MS_(k) immediately after data transmission from the BSto the RS and MS_(k), for example at the first scheduling instant 102.

The BS selects a pair of MSs, for example MS_(i) and MS_(j) in a certainscheduling epoch and transmits data to them jointly by superpositioncoding. Notably, the scheduling epoch features simultaneous unicast toboth MSs, rather than broadcast to them. That is, each user receivesdifferent data from the jointly transmitted data and completelydifferent scheduling algorithms can be used for the two users.Obviously, the Multi-User Superpositioning (MUS) of the presentinvention is applicable to the case where one or more users are selectedper channel. For convenience sake, the case where two users exist isconsidered herein. Although many classes of scheduling algorithms can beused for the present invention, embodiments of the present inventionwill be described based on a standard scheduling algorithm.

FIG. 2 illustrates a downlink transmission scenario according to anembodiment of the present invention.

Referring to FIG. 2, a BS 202 supports downlink transmission to bothusers, MS₁ 206 and MS₂ 208. The link 214 between the BS 202 and an RS204 has a very high and stable SNR denoted by γ_(R) [dB]. MS₁ has a gooddirect link 212 to the BS 202, with an SNR equal to γ₀. Although MS₁ isserved via the direct link 212, it also has a good direct link 216 tothe RS 204 with an SNR of γ₂₁.

MS₂ has a bad direct link 210 to the BS 202 due to shadowing, forexample. So it needs to be supported by the RS 204 in order to receivedownlink transmissions from the BS 202. The RS 204 has a direct link 218to MS₂ 208, with an SNR of γ₂₂. For instance, if the SNR of the directlink 218, γ₂₂ is lower than a minimum SNR (i.e. threshold) required fordata reception from the BS 202, the direct link 218 is determined to bebad.

Relaying with superposition coding brings an increased spectralefficiency for MS₁ under the condition of Equation (1):

γ₂₁≧γ₀   (1)

However, the increase in spectral efficiency is not significant if γ₂₁is close to γ₀. In an embodiment of the present invention describedlater, an MUS scheme is proposed which can gain an increase in thespectral efficiency or a wide range of values γ₂₁. The MUS scheme isdivided into two steps.

In Step 1, during a time of N symbols, i.e. an N -symbol duration, theBS 202 transmits the following superposition-coded data according toEquation (2):

√{square root over (1−α)}X ₁ √{square root over (αX₂)}, 0≦α≦1   (2)

where X₁ denotes an N-symbol message carrying D₁ information bits forMS₁ 206, X₂ denotes an N-symbol message carrying D₂ information bits forMS₂ 208, and α is a coefficient appropriately selected between 0 and 1.For the RS 204 to be able to decode both messages, the followingrestrictions are put to the transmission rates R₁ and R₂ of the messagesX₁ and X₂., shown in Equation (3):

$\begin{matrix}{R_{1} \leq {{\log_{2}\left( {1 + \frac{\left( {1 - \alpha} \right)\gamma_{R}}{1 + {\alpha \; \gamma_{R}}}} \right)}\mspace{25mu} R_{2}} \leq {\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}} & (3)\end{matrix}$

Via the direct link 212 from the BS 202, MS₁ 206 receives the signal asshown in Equation (4):

γ₁₁ =h ₀(√{square root over (1−α)}X ₁ +√{square root over (α)}X ₂)+z ₁₁  (4)

where h₀ denotes a channel impulse response representing the channelcharacteristics between MS₁ 206 and the BS 202 and z₁₁ denotes othersignals (e.g. noise) introduced to MS₁ 206. Then MS₁ 206 waits for Step2, i.e. transmission of the RS 204. On the other hand, having the baddirect link 210 to the BS 202, MS₂ 208 completely ignores thetransmission from the BS 202.

In Step 2, during M symbols following to the N symbols of Step 1, i.e.during the next M-symbol duration, the RS 204 transmits a message X₃carrying the D₂ information bits for MS₂ 208 at a rate R_(s2), accordingto Equation (5):

R _(s2)≦log₂(1+min(γ₂₁,γ₂₂))   (5)

so that both MS₁ 206 and MS₂ 208 are able to receive it from the RS 204and decode it. For easier notation, as shown in Equation (6):

γ₂=min(γ₂₁,γ₂₂)   (6)

To use maximal possible rates in Step 1 and Step 2, the followingrelationship of Equation (7) should be placed

$\begin{matrix}{{R_{2}N} = {\left. {R_{s\; 2}M}\Leftrightarrow M \right. = {N\frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}}} & (7)\end{matrix}$

After the transmission of the RS 204, MS₂ 208 decodes the intendedunicast message X of D₂ bits. MS₁ 206 also decodes the message X,creates the D₂ bits intended for MS₂ 208 out of that message X, andreconstructs the message X₂ using the D₂ information bits. Then MS₁ 206acquires the message X₁ by subtracting the components of the message X₂from the received signal described as Equation (4) according to thefollowing equation. Thus, MS₁ 206 achieves the intended D₁ informationbits by decoding the message X₁., according to Equation (8):

γ₁₂=γ₁₁ −h ₀ √{square root over (α)}X ₂ =h ₀√{square root over (1−α)}X ₁+z ₁₁   (8)

The transmission rate R₁ of the message X₁ satisfies the condition ofEquation (9):

$\begin{matrix}{{R_{1} \leq {\log_{2}\left( {1 + {\left( {1 - \alpha} \right)\gamma_{0}}} \right)}},\mspace{25mu} {\gamma_{0} = \frac{{h_{0}}^{2}}{\sigma^{2}}}} & (9)\end{matrix}$

where σ² denotes Gaussian noise power. Equation (3) describes thetransmission rates of the messages X₁ and X₂ received in combination atthe RS 204 and Equation (9) describes the transmission rate of datareceived at MS₁ 206 after interference cancellation.

Hence, the total transmission rate of this system can be calculated asshown in Equation (10):

$\begin{matrix}{R_{MUS} = {\frac{N\left( {R_{1} + R_{2}} \right)}{N + M} = \frac{R_{1} + R_{2}}{1 + \frac{R_{2}}{R_{s\; 2}}}}} & (10)\end{matrix}$

FIG. 3 is a flowchart illustrating a joint unicast operation accordingto an embodiment of the present invention. Steps 302 through 314 can beperformed by the BS or by another network node above the BS. Forconvenience' sake, it will be described that these steps are performedin the BS.

Referring to FIG. 3, the BS acquires given parameters, i.e. the numberof users to be served, K and SNRs representing the channel gains of theRS and the BS with respect to the individual users in step 302. K isequal to the number of MSs that intend to receive data within theservice area of the BS and the SNRs are obtained from channel stateinformation measured and reported to the BS in measurement reports bythe MSs. Also, the BS sets the number of transmission symbols, N, thatit will use at each scheduling instant.

In step 304, the BS generates all possible user pairs associated withthe K users and selects a first user pair k (k=1). Herein,

$1 \leq k \leq {\frac{K\left( {K - 1} \right)}{2}.}$

Let the MS having the higher SNR of a BS-RS link be denoted by MS₁ andthe other MS be denoted by MS₂. Then, the BS checks the SNR between eachMS and the RS in step 306. Specifically, the MS₁-RS link has an SNR ofγ₂₁ and the MS₂-RS link has an SNR of γ₂₂. The smaller SNR between themis denoted by γ₂. As stated before, the BS-RS SNR is γ_(R).

In step 308, the BS calculates the transmission rates R₁ and R₂ for MS₁and MS₂ with respect to the given coefficient α in Step 1 according toEquation (11):

R ₁=log₂(1+(1−α)γ₀)

R₂=log₂(1+αγ_(R))   (11)

Then, the BS calculates overall rates r₁ and r₂ for MS₁ and MS₂ at thescheduling instant by Equation (12). Note that the sum of r₁ and r₂ isequal to R_(MUS) described in Equation (10).

$\begin{matrix}{{r_{1} = \frac{\log_{2}\left( {1 + {\left( {1 - \alpha} \right)\gamma_{0}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}}{r_{2} = \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}}} & (12)\end{matrix}$

The BS decides an overall scheduling objective function for thescheduling instant using the overall rates r₁ and r₂ by a knownscheduling algorithm in step 310. The overall scheduling objectivefunction depends on the used scheduling algorithm. It outputs ascheduling gain for the input of the overall rates r₁ and r₂ and whenneeded, other parameters according to the scheduling algorithm. Forexample, the scheduling algorithm can use proportional fairnessmeasurements as a criterion. Then the BS determines a coefficient α_(k)maximizing the scheduling gain for the user pair k and determines ascheduling gain G_(k) corresponding to the coefficient α_(k).

If the next user pair remains in step 312, the BS selects the next userpair k (k=k+1) and returns to step 306. In the absence of the next userpair, the BS selects a user pair k having a maximal G_(k) and acoefficient α_(k) corresponding to the maximal G_(k) in step 314 andtransmits data to the selected user pair in step 316. The specificoperation of step 316 is described in FIG. 4.

Referring to FIG. 4, the BS transmits a message X₁ of N symbols carryinginformation bits for MS₁ and a message X₂ of N symbols carryinginformation bits for MS₂ using the coefficient α_(k) selected in step314 of FIG. 3 by superposition coding described in Equation (2) in step402. The message X₁ includes NR₁=D₁ information bits according to thetransmission rate R₁ calculated in step 308 and the message X₂ includesNR₂=D₂ information bits according to the transmission rate R₂ calculatedin step 308. The superposition-coded data is delivered to MS₁ having agood direct link to the BS.

In step 404, the RS acquires the two messages X₁ and X₂ by decoding thesuperposition-coded data and transmits the D₂ information bits extractedfrom the message X₂ in M transmission symbols at the transmission rateR_(s2) calculated by Equation (5). R_(s2) and M are computed by Equation(13) below. Both MS₁ and MS₂ are able to receive the data from the RS.

$\begin{matrix}{{R_{s\; 2} = {\log_{2}\left( {1 + \gamma_{2}} \right)}}{M = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}}} & (13)\end{matrix}$

In step 406, MS₁ receives the superposition-coded data from the BS andthe data from the RS, acquires the message X₁ by subtracting thecomponents of the message X₂ from the superposition-coded data referringto the D₂ information bits resulting from decoding the data receivedfrom the RS, and then achieves the intended D₁ information bits bydecoding the message X₁.

Now a description will be made of the performances of the MUS schemeaccording to the embodiment of the present invention in many scenarios.In the following description, a conventional scheduling strategy isconsidered.

FIG. 5 is a graph comparing the present invention with conventionalmethods in communication performance expressed as normalized rates, whenequal time resources are allocated to two MSs.

The two MSs, MS₁ and MS₂, can use N₁ transmission symbols, respectively.The BS transmits information bits in N₁ transmission symbols directly toMS₁ and in N₁ transmission symbols to MS₂ via the RS. Specifically, fora duration of N₁ transmission symbols, the BS transmits the informationbits for MS₁ at a rate shown in Equation (14):

R _(c1)=log₂(1+γ₀)   (14)

For a duration of N₁ symbols, the BS and the RS transmit the informationbits to MS₂ in a multi-hop manner at a rate shown in Equation (15):

$\begin{matrix}{R_{c\; 2} = \frac{{\log_{2}\left( {1 + \gamma_{R}} \right)}{\log_{2}\left( {1 + \gamma_{22}} \right)}}{{\log_{2}\left( {1 + \gamma_{R}} \right)} + {\log_{2}\left( {1 + \gamma_{22}} \right)}}} & (15)\end{matrix}$

For a conventional method without superposition coding, the overall datarate is given by Equation (16):

$\begin{matrix}{R_{conv} = \frac{R_{c\; 1} + R_{c\; 2}}{2}} & (16)\end{matrix}$

When the N₁ symbols are transmitted to MS₁, another conventional methodachieves a transmission rate R_(srd2) as shown in Equation (17):

$\begin{matrix}{R_{{srd}\; 2} = \frac{{\log_{2}\left( {1 + \gamma_{R}} \right)}{\log_{2}\left( {1 + \gamma_{22}} \right)}}{{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)} + {\log_{2}\left( {1 + \gamma_{22}} \right)}}} & (17)\end{matrix}$

where srd represents source, relay, and destination.

The present invention uses the MUS scheme for 2N, symbols and itsoverall transmission rate is determined by using Equation (10). For theMUS scheme, the coefficient α is computed by Equation (18):

$\begin{matrix}{\frac{\left( {1 - \alpha} \right)\gamma_{R}}{1 + {\alpha \; \gamma_{R}}} = {\left. {\left( {1 - \alpha} \right)\gamma_{0}}\Rightarrow\alpha \right. = {\frac{1}{\gamma_{0}} - \frac{1}{\gamma_{R}}}}} & (18)\end{matrix}$

where if it is assumed that γ_(R)>γ₀, α is positive. In this case,substitution of Equation (18) into Equation (13) leads to Equation (19):

$\begin{matrix}\begin{matrix}{{R_{1} = {\log_{2}\left( {\frac{\gamma_{0}}{\gamma_{R}} + \gamma_{0}} \right)}};} & {R_{2} = {\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}} \\{{{R_{1} + R_{2}} = {\log_{2}\left( {1 + \gamma_{R}} \right)}},} & {R_{s\; 2} = {\log_{2}\left( {1 + \gamma_{2}} \right)}}\end{matrix} & (19)\end{matrix}$

Then the overall rate for the MUS scheme is given by Equation (20):

$\begin{matrix}{R_{MUS} = \frac{\log_{2}\left( {1 + \gamma_{R}} \right)}{1 + \frac{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}} & (20)\end{matrix}$

In FIG. 5, two groups of curves are illustrated, one having γ₂₁=γ₂₂=2γ₀and the other having γ₂₁=γ₂₂=γ_(R)=30 dB. When γ₂₁ and γ₂₂ are fixed, itis noted that the MUS scheme of the present invention with R_(MUS) issuperior to the two other conventional methods with R_(conv) andR_(srd2) and the improvement becomes better when γ₀ increases. Also, theincrease of the value of γ₂ makes the improvement brought by the MUSmore significant. The two curves for R_(MUS) converge at γ₀=30 dB,although the curves have different values γ₂ at this point. This isexplained by the fact that when γ₀=γ_(R), the data for MS₂ does notexist i.e. D₂=0.

FIG. 6 is another graph comparing the present invention with theconventional methods in communication performance expressed asnormalized rates, when equal time resources are allocated to two MSs andγ₂₁≠γ₂₂. For each of the present invention and the conventional methods,two curves are illustrated for the two cases of γ₂₁=γ₂=2γ₀, γ₂₂=5γ₀ andγ₂₁=2γ₀, γ₂₂=γ₂=0.5*γ₀. As noted from FIG. 6, the transmission rateR_(MUS) of the present invention is beneficial in terms of improvingfairness between two users.

FIG. 7 is a graph comparing the present invention with the conventionalmethods in communication performance expressed as normalized rates, whena transmission rates are allocated at a predetermined ratio to two MSs.Two groups of curves are illustrated, one having γ₂₁=γ₂₂=2γ₀ and theother having γ₂₁=γ₂₂=γ_(R)=30 dB.

The numbers of information bits intended for MS₁ and MS₂ are D₁ and D₂,respectively, and the transmission rates for the MSs should bedetermined by finding the time that is needed to transfer the data tothem. D₁ and D₂ are computed by Equation (21):

$\begin{matrix}{{D_{1} = {N_{1}{\log_{2}\left( {\frac{\gamma_{0}}{\gamma_{R}} + \gamma_{0}} \right)}}};\mspace{14mu} {D_{2} = {N_{1}{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}}};\mspace{14mu} {\alpha = {\frac{1}{\gamma_{0}} - \frac{1}{\gamma_{R}}}}} & (21)\end{matrix}$

The total symbol duration consumed to send the data is shown in Equation(22):

$\begin{matrix}{{{N_{1} + N_{2}},{where}}{N_{2} = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}}} & (22)\end{matrix}$

Then the ratio between the transmission rate R_(MS1) for MS₁ and thetransmission rate R_(MS2) for MS₂ is computed by Equation (23):

$\begin{matrix}{\frac{R_{{MS}\; 1}}{R_{{MS}\; 2}} = \frac{\log_{2}\left( {\frac{\gamma_{0}}{\gamma_{G}} + \gamma_{0}} \right)}{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}} & (23)\end{matrix}$

The BS now determines the numbers of transmission symbols that areneeded to transfer the information bits D₁ and D₂ by direct transmissionto MS₁ and multi-hop transmission to MS₂. The number of symbols neededfor the direct transmission, shown in Equation (24), is M₁ where

$\begin{matrix}{M_{1} = {N_{1}\frac{\log_{2}\left( {\frac{\gamma_{0}}{\gamma_{R}} + \gamma_{0}} \right)}{\log_{2}\left( {1 + \gamma_{0}} \right)}}} & (24)\end{matrix}$

On the other hand, the number of symbols consumed by the multi-hoptransmission to send D₂ bits to MS₂ is shown in Equation (25):

$\begin{matrix}{M_{2} = {N_{1}\left( {\frac{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}{\log_{2}\left( {1 + \gamma_{R}} \right)} + \frac{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}{\log_{2}\left( {1 + \gamma_{22}} \right)}} \right)}} & (25)\end{matrix}$

FIG. 7 shows that the MUS scheme of the present invention with R_(MUS)is superior to the two other conventional methods with R_(conv′) andR_(srd2′), when the ratio between the transmission rates of the two MSsis preset.

FIG. 8 is another graph comparing the present invention with theconventional methods in communication performance expressed asnormalized rates, when a transmission rates are allocated at apredetermined ratio to two MSs and γ₂₁≠γ₂₂. For each of the presentinvention and the conventional methods, two curves are illustrated forthe two cases of γ₂₁=γ₂=2γ₀, γ₂₂=5γ₀ and γ₂₁=2γ₀, γ₂₂=γ₂=0.5*γ₀. Asnoted from FIG. 8, even when γ₂₁≠γ₂₂, the transmission rate R_(MUS) ofthe present invention offers much higher gains than in the conventionalmethods, as far as γ₀ is sufficiently high.

FIG. 9 is a graph comparing the present invention with the conventionalmethods in communication performance expressed as normalized rates, whenequal transmission rates are allocated to two MSs. Two different cases,Case 1 and Case 2 are considered herein. Case 1 satisfies Equation (26):

$\begin{matrix}{\gamma_{0} > \frac{\gamma_{R}}{\sqrt{1 + \gamma_{R}}}} & (26)\end{matrix}$

In this case, the coefficient α is selected according to Equation (27)such that the basic and the superposition-coded stream from the BS haveequal rates. This means that, after the transmission of the RS, both MS₁and MS₂ should be able to receive the same amount of data over the sametime.

$\begin{matrix}{{\frac{\left( {1 - \alpha} \right)\gamma_{R}}{1 + {\alpha \; \gamma_{R}}} = {\alpha \; \gamma_{R}}},\mspace{25mu} {\alpha = \frac{\sqrt{1 + \gamma_{R}} - 1}{\gamma_{R}}}} & (27)\end{matrix}$

With such a choice of α, the message X₁ can be decoded by MS₁ after thetransmission of the RS, since the following condition is satisfied inEquation (28):

$\begin{matrix}{{{{if}\mspace{14mu} \gamma_{0}} > \frac{\gamma_{R}}{\sqrt{I + \gamma_{R}}}},{{{then}\mspace{14mu} \left( {1 - \alpha} \right)\gamma_{0}} > \frac{\left( {1 - \alpha} \right)\gamma_{R}}{1 + {\alpha \; \gamma_{R}}}}} & (28)\end{matrix}$

As a consequence, the overall rate achieved by the MUS scheme is foundto be

$\begin{matrix}{R_{MUS} = {\frac{\log_{2}\left( {1 + \gamma_{R}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}} = \frac{\log_{2}\left( {1 + \gamma_{R}} \right)}{1 + {\frac{1}{2}\frac{\log_{2}\left( {1 + \gamma_{R}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}}}} & (29)\end{matrix}$

Case 2 satisfies Equation (30):

$\begin{matrix}{\gamma_{0} \leq \frac{\gamma_{R}}{\sqrt{1 + \gamma_{R}}}} & (30)\end{matrix}$

In this case, the coefficient α is selected according to Equation (31):

(1−α)γ₀=αγ_(R), α=γ₀/γ₀+γ_(R)   (31)

The amount of data decoded by MS1 after Step 1 and Step 2 is equal tothat amount of data that RS decodes for MS₂ in Step 1. After RS forwardsthat data to MS₂ and MS₁, both MSs extract exactly the same amount ofdata, thus achieving equal rates after the two steps of the presentinvention. Under the condition of Equation (30), the RS can easilydecode the message X₁ intended for MS₁ received from the BS. As aconsequence, the overall rate achieved by the MUS scheme is found inEquation (32) to be

R _(MUS)=2 log₂(1+γ₀)/1+log₂(1+αγ_(R))/log₂(1+γ₂)   (32)

In the scenarios of FIG. 9, since the transmission rates R_(MS1) andR_(MS2) of MS1 and MS2 are equal, Equation (33) is satisfied:

R _(MS1) =R _(MS2)=1/2R _(MUS)   (33)

Under the condition of Equation (33), the number N₁ of symbolstransmittable to MS₁ via the direct link from the BS and the number N₂of symbols transmittable to MS₂ via the RS are placed in therelationship of Equation (34):

N ₁ log₂(1+γ₀)=N ₂ log₂(1+γ_(R))·log₂(1+γ₂₂)/log₂(1+γ₂)+log₂(1+γ_(R))  (34)

Referring to FIG. 9, two groups of curves are illustrated, one havingγ₂₁=γ22=2γ₀ and the other having γ₂₁=γ₂₂=γ_(R)=30 dB. As noted from thecurves, there are significant regions of γ₀ where the MUS scheme offershigher rates.

FIG. 10 is another graph comparing the present invention with theconventional methods in communication performance expressed asnormalized rates, when equal transmission rates are allocated to twoMSs. For each of the present invention and the conventional methods, twocurves are illustrated for the two cases of γ₂₁=γ₂=2γ₀, γ₂₂=5γ₀ andγ₂₁=2γ₀, γ₂₂=γ₂=0.5*γ₀. As noted from FIG. 10, the MUS scheme of thepresent invention with the transmission rate R_(MUS) relativelyoutperforms the conventional methods with the transmission ratesR_(conv′) and R_(srd2′).

A description will now be made of exemplary operations based on the MUSscheme according to the embodiment of the present invention. It isassumed that the BS schedules the users according to some criterion i.e.proportional fairness or maximal rate.

In a first operation, the BS decides to serve MS_(i) over a direct linkwith γ_(0i). Then the BS attempts to find MS_(j) such that Equation (35)is satisfied

$\begin{matrix}{\gamma_{2j} = {\min\limits_{k \neq i}{{\gamma_{2k} - \gamma_{2i}}}}} & (35)\end{matrix}$

After identifying MS_(j), the BS determines whether there is a rateimprovement if the MUS scheme is applied for MS_(i) and MS_(j). If therate improvement can be achieved, the BS transmits data to the two MSsaccording to the MUS scheme by tuning the coefficient α so as to satisfya predetermined rate criterion.

In a second operation, the BS decides to serve MS_(i) over a multi-hoplink. Then, the BS attempts to find MS_(j) that has γ_(2j) close toγ_(2i), but also a very good link γ₀ _(j). γ_(2i) is the SNR of themulti-hop link to MS_(i) and γ_(2j) is the SNR of the multi-hop link toMS_(j). In case there are several candidates {j}, then the BS cancalculate the rates achievable for each user {j}. If there is a usercandidate {j} having a maximal rate than a predetermined transmissionrate, the BS sends data to the user by using the MUS scheme.

Now a description will be made of the case where there are K users inthe system and the system throughput should be maximized. At eachscheduling instant, the BS calculates a rate achieved by scheduling auser pair MS_(i) and MS_(j). Here it calculates the overall rates of theMSU scheme and other transmission schemes. Finally, the BS selects atransmission scheme and a pair of users or a single user that maximizethe overall rate and transmits data to the selected user pair or singleuser in the selected transmission scheme.

As is apparent from the above description, the present inventionadvantageously improves communication quality and increases datathroughput by providing a multi-hop link via an RS between a BS and anMS in addition to a good direct link between them, if possible.

While the invention has been shown and described with reference tocertain preferred embodiments of the present invention thereof, it willbe understood by those skilled in the art that various changes in formand details may be made therein without departing from the spirit andscope of the present invention as defined by the appended claims andtheir equivalents.

1. A method for transmitting data via a Relay Station (RS) in a wirelessrelay system, comprising the steps of: encoding a first data message anda second data message directed to a first Mobile Station (MS) and asecond MS scheduled at a current scheduling instant by superpositioncoding by a Base Station (BS), the first data message carrying a firstinformation bit stream for the first MS, the second data messagecarrying a second information bit stream for the second MS, the first MShaving a relatively good direct link to the BS, and the second MS havinga relatively bad direct link to the BS; transmitting thesuperposition-coded data to the first MS and the RS connected betweenthe BS and the first and second MSs by the BS; receiving thesuperposition-coded data from the BS; extracting the second informationbit stream for the second MS by decoding the superposition-coded data;and transmitting a third data message carrying the second informationbit stream to the first and second MSs by the RS.
 2. The method of claim1, wherein the BS generates the superposition-coded data using a givencoefficient for the first and second MSs by:√{square root over (1−α)}X ₁ +√{square root over (α)}X ₂, 0≦α≦1, where αis the given coefficient and X₁ and X₂ are the first and second datamessages, respectively.
 3. The method of claim 1, further comprisingdetermining transmission rates for the first and second messages using agiven coefficient for the first and second MSs by:R ₁=log₂(1+(1−α)γ₀)R ₂=log₂(1+αγ_(R)), where α is the given coefficient between 0 and 1, R₁and R₂ are the transmission rates of the messages X₁ and X₂, γ₀ is aSignal-to-Noise Ratio (SNR) of the direct link between the BS and thefirst MS, and γ_(R) is an SNR of a link between the BS and the RS. 4.The method of claim 3, further comprising: calculating overall rates forall MS pairs of MSs served by the BS using the transmission rates at thescheduling instant by:$r_{1} = \frac{\log_{2}\left( {1 + {\left( {1 - \alpha} \right)\gamma_{0}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}$$r_{2} = \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}$where r₁ and r₂ are the overall rates of two MSs of each of the userpairs; calculating a scheduling gain for the overall rates of the eachuser pair by a predetermined scheduling algorithm; and determining totransmit data to a user pair having a maximal scheduling gain among alluser pairs.
 5. The method of claim 1, further comprising: calculatingtransmission rates for data messages directed to two MSs of each of allMS pairs of MSs served by the BS using a given coefficient for the eachuser pair; and calculating the number of bits for each of the first andsecond information bit streams according to the transmission rates andthe number of transmission symbols used by the BS at the schedulinginstant by:D₁=NR₁D₂=NR₂ where D₁ is the number of bits for the first information bitstream, D₂ is the number of bits for the second information bit stream,N is the number of transmission symbols used by the BS, and R₁ and R₂are the transmission rates of the data messages.
 6. The method of claim1, further comprising determining the number of transmission symbols anda transmission rate for the third data message by:R_(s 2) = log₂(1 + γ₂)$M = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}$ whereR_(s2) is the transmission rate of the third data message, M is thenumber of transmission symbols to be used for the third data message, γ₂is a smaller SNR between the SNRs of links between the RS and the firstand second MSs, and D₂ is the number of bits for the second informationbit stream.
 7. A method for receiving data via a Relay Station (RS) in awireless relay system, comprising the steps of: receivingsuperposition-coded data including a first data message and a seconddata message by a first Mobile Station (MS) scheduled along with asecond MS at a current scheduling instant by a Base Station (BS), thefirst data message carrying a first information bit stream for the firstMS, the second data message carrying a second information bit stream forthe second MS, the first MS having a relatively good direct link to theBS, and the second MS having a relatively bad direct link to the BS;receiving a third data message carrying the second information bitstream directed to the second MS from the RS connected between the BSand the first and second MSs; extracting the second information bitstream by decoding the third data message by the first MS; and acquiringthe first data message by removing components related to the secondinformation bit stream from the superposition-coded data and extractingthe first information bit stream by decoding the first data message bythe first MS.
 8. The method of claim 7, wherein the superposition-codeddata is formed using a given coefficient for the first and second MSsby:√{square root over (1−α)}X ₁ +√{square root over (α)}X ₂, 0≦α≦1 where αis the given coefficient and X₁ and X₂ are the first and second datamessages, respectively.
 9. The method of claim 7, wherein the first datamessage is acquired using a given coefficient for the first and secondMSs, by:γ₁₂=γ₁₁ −h ₀ √{square root over (α)}X ₂ =h ₀√{square root over (1−α)}X ₁+z ₁₁ where γ₁₂ is data including the first data message, γ₁₁ is thesuperposition-coded data, α is the given coefficient, X₁ and X₂ are thefirst and second data messages, respectively, h₀ is a channel impulseresponse representing channel characteristics of the direct link betweenthe BS and the first MS, and z₁₁ is noise.
 10. The method of claim 7,wherein transmission rates of the first and second messages aredetermined using a given coefficient for the first and second MSs by:R ₁=log₂(1+(1−α)γ₀)R ₂=log₂(1+αγ_(R)) where α is the given coefficient between 0 and 1, R₁and R₂ are the transmission rates of the messages X₁ and X₂, γ₀ is aSignal-to-Noise Ratio (SNR) of the direct link between the BS and thefirst MS, and γ_(R) is an SNR of a link between the BS and the RS. 11.The method of claim 7, wherein the number of bits for each of the firstand second information bit streams is determined according totransmission rates of the first and second data messages and the numberof transmission symbols used by the BS at the scheduling instant, by:D₁=NR₁D₂=NR₂ where D₁ is the number of bits for the first information bitstream, D₂ is the number of bits for the second information bit stream,N is the number of transmission symbols used by the BS, and R₁ and R₂are the transmission rates of the data messages.
 12. The method of claim7, wherein the number of transmission symbols and a transmission ratefor the third data message are determined by: R_(s 2) = log₂(1 + γ₂)$M = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}$ whereR_(s2) is the transmission rate of the third data message, M is thenumber of transmission symbols to be used for the third data message, γ₂is a smaller SNR between the SNRs of links between the RS and the firstand second MSs, and D₂ is the number of bits for the second informationbit stream.
 13. The method of claim 7, further comprising receiving thethird data message and extracting the second information bit stream bydecoding the third data message by the second MS.
 14. An apparatus fortransmitting and receiving data via a Relay Station (RS) in a wirelessrelay system, comprising: a Base Station (BS) for encoding a first datamessage and a second data message directed to a first Mobile Station(MS) and a second MS, scheduled at a current scheduling instant bysuperposition coding, the first data message carrying a firstinformation bit stream for the first MS, the second data messagecarrying a second information bit stream for the second MS, and thesecond MS having a relatively bad direct link to the BS, andtransmitting the superposition-coded data; and the RS connected betweenthe BS and the first and second MSs, for establishing multi-hop links,receiving the superposition-coded data from the BS, extracting thesecond information bit stream for the second MS by decoding thesuperposition-coded data, and transmitting a third data message carryingthe second information bit stream to the first and second MSs, whereinthe first MS receives the superposition-coded data from the BS,receiving the third data message from the RS, extracting the secondinformation bit stream by decoding the third data message, acquiring thefirst data message by removing components related to the secondinformation bit stream from the superposition-coded data, and extractingthe first information bit stream by decoding the first data message. 15.The apparatus of claim 14, wherein the superposition-coded data isformed using a given coefficient for the first and second MSs by:√{square root over (1−α)}X _(1+√){square root over (α)}X ₂, 0≦α≦1 whereα is the given coefficient and X₁ and X₂ are the first and second datamessages, respectively.
 16. The apparatus of claim 14, whereintransmission rates of the first and second messages are determined usinga given coefficient for the first and second MSs by:R ₁=log₂(1+(1−α)γ₀)R ₂=log₂(1+αγ_(R)) where α is the given coefficient between 0 and 1, R₁and R₂ are the transmission rates of the messages X₁ and X₂, γ₀ is aSignal-to-Noise Ratio (SNR) of the direct link between the BS and thefirst MS, and γ_(R) is an SNR of a link between the BS and the RS. 17.The apparatus of claim 16, wherein the BS calculates overall rates forall MS pairs of MSs served by the BS using the transmission rates at thescheduling instant by:$r_{1} = \frac{\log_{2}\left( {1 + {\left( {1 - \alpha} \right)\gamma_{0}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}$$r_{2} = \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha \; \gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}$where r₁ and r₂ are the overall rates of two MSs of each of the userpairs, calculates a scheduling gain for the overall rates of the eachuser pair by a predetermined scheduling algorithm, and determines totransmit data to a user pair having a maximal scheduling gain among alluser pairs.
 18. The apparatus of claim 14, wherein the number of bitsfor each of the first and second information bit streams are calculatedaccording to the transmission rates and the number of transmissionsymbols used by the BS at the scheduling instant by:D₁=NR₁D₂=NR₂ where D₁ is the number of bits for the first information bitstream, D₂ is the number of bits for the second information bit stream,N is the number of transmission symbols used by the BS, and R₁ and R₂are the transmission rates of the data messages.
 19. The apparatus ofclaim 14, wherein the number of transmission symbols and a transmissionrate for the third data message are determined by:R_(s 2) = log₂(1 + γ₂)$M = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}$ whereR_(s2) is the transmission rate of the third data message, M is thenumber of transmission symbols to be used for the third data message, γ₂is a smaller SNR between the SNRs of links between the RS and the firstand second MSs, and D₂ is the number of bits for the second informationbit stream.
 20. The apparatus of claim 14, wherein the first MS acquiresthe first data message using a given coefficient for the first andsecond MSs, by:γ₁₂=γ₁₁ −h ₀ √{square root over (α)}X ₂ =h ₀√{square root over (1−α)}X ₁+z ₁₁ where γ₁₂ is data including the first data message, γ₁₁ is thesuperposition-coded data, α is the given coefficient, X₁ and X₂ are thefirst and second data messages, respectively, h₀ is a channel impulseresponse representing channel characteristics of the direct link betweenthe BS and the first MS, and z₁₁ is noise.
 21. The apparatus of claim14, wherein the second MS receives the third data message and extractingthe second information bit stream by decoding the third data message.